It was the study of the phenomena of polarization which led Fresnel to his bold conception of transverse vibrations, and subsequently induced him to penetrate further into the constitution of the ether.  We know the experiment of Arago on the noninterference of polarized rays in rectangular planes.  While two systems of waves, proceeding from the same source of natural light and propagating themselves in nearly parallel directions, increase or become destroyed according to whether the nature of the superposed waves are of the same or of contrary signs, the waves of the rays polarized in perpendicular planes, on the other hand, can never interfere with each other.  Whatever the difference of their course, the intensity of the light is always the sum of the intensity of the two rays.

Fresnel perceived that this experiment absolutely compels us to reject the hypothesis of longitudinal vibrations acting along the line of propagation in the direction of the rays.  To explain it, it must of necessity be admitted, on the contrary, that the vibrations are transverse and perpendicular to the ray.  Verdet could say, in all truth, “It is not possible to deny the transverse direction of luminous vibrations, without at the same time denying that light consists of an undulatory movement.”

Such vibrations do not and cannot exist in any medium resembling a fluid.  The characteristic of a fluid is that its different parts can displace themselves with regard to one another without any reaction appearing so long as a variation of volume is not produced.  There certainly may exist, as we have seen, certain traces of rigidity in a liquid, but we cannot conceive such a thing in a body infinitely more subtle than rarefied gas.  Among material bodies, a solid alone really possesses the rigidity sufficient for the production within it of transverse vibrations and for their maintenance during their propagation.

Since we have to attribute such a property to the ether, we may add that on this point it resembles a solid, and Lord Kelvin has shown that this solid, would be much more rigid than steel.  This conclusion produces great surprise in all who hear it for the first time, and it is not rare to hear it appealed to as an argument against the actual existence of the ether.  It does not seem, however, that such an argument can be decisive.  There is no reason for supposing that the ether ought to be a sort of extension of the bodies we are accustomed to handle.  Its properties may astonish our ordinary way of thinking, but this rather unscientific astonishment is not a reason for doubting its existence.  Real difficulties would appear only if we were led to attribute to the ether, not singular properties which are seldom found united in the same substance, but properties logically contradictory.  In short, however odd such a medium may appear to us, it cannot be said that there is any absolute incompatibility between its attributes.